The invention relates to the general field of liquid crystal devices (LCDs) with particular reference to novel uses of the black matrix.
FIG. 1 is a schematic plan view of a LCD (liquid crystal display) 1 having an array of light valves 2, and row and column addressing lines 3. FIG. 1 shows a 5xc3x975 array of light valves, but typically the LCD display 1 comprises up to 1,200xc3x971,000 light valves 2 and associated addressing lines 3. The valves are shown as square arrays but it is to be understood that other shapes, such as rectangles, can be used. In this example, the LCD display is a transmissive type arranged to selectively allow light through the array depending upon the state of each light valve.
FIG. 2 is a schematic cross-sectional view of the LCD display 1. This display optionally comprises upper and lower outer substrates 10 of a suitable transparent material, such as glass or, for the plate on which the TFT (thin film transistor) is formed, preferably quartz, at a separation of about 1 to 5 xcexcm. The space between the quartz plates 10 is sealed such as by an epoxy sealant (not shown) and filled with a liquid crystal layer 20, suitably a ferroelectric material or a twisted nematic material. Driving circuitry is carried by the plates 10. One of the plates 10 carries a large, transparent sheet electrode 21 such as indium tin oxide (ITO) that may be coupled to a reference potential such as ground. The other quartz plate 10 carries a driving circuitry layer 30, including a regular array of smaller transparent sheet conductors 2, that define the locations of the pixels of the display, each being connected to a TFT 4 (connections not shown) which is accessed through the row and column addressing lines 3.
In use, a TFT is selectively activated by addressing the row and column addressing lines 3, either from an external circuit or from control circuitry located on chip, in order to change the light transmission properties of the liquid crystal layer 20 in that region, thus forming a light valve corresponding to one pixel of an image.
Another substructure commonly used in LCDs is a black matrix film. The black matrix serves to block out any light that might otherwise leak out through the spaces between pixels, such as 25 in FIG. 2.
The black matrix comprises a grid of opaque elements 27 located, with some overlap, between the light valves 2. Areas directly below the black matrix are where the individual TFTs and row-column addressing lines are located. Although much of the TFT area is itself opaque, there is no guarantee that all light emission between will be blocked since the various parts of the TFTs and row-column addressing lines are irregularly shaped and do not always overlap. Additionally, it is desired to overlap the light valve 2 so as to block out light from its edge where the liquid crystal may not fully block the light because of the non-uniform electric field at the edges of sheet conductors 2. Also, it is desired to shield related portions of the circuitry from visible light and UV radiation in order to minimize possible photoconductivity effects on the TFTs.
In prior art devices (such as shown in FIG. 2) the black matrix 27 is located on the opposite plate to the TFT devices 4. In order to minimize light scattering across the gap between the plates, it needs to be wider than this gap. This in turn, however, reduces the aperture ratio. This is much more of a problem for polysilicon displays used in digital projectors as the pixel size (15-30 microns) is much smaller than in amorphous silicon displays (100-200 microns).
The prior art teaches that the black matrix elements may be either conductive or insulating. Examples of the latter may be found in Kwon et al U.S. Pat. No. 5,866,919 and Kwon et al. U.S. Pat. No. 5,926,702, but in these cases the black matrix was used as part of the phosphor plate rather than being in the TFT plane of the liquid crystal device. Furthermore, black matrices of this type are often found to be unsuitable due to, for example, unpredictable photoconductivity effects and capacitive coupling effects, particularly in neighboring semiconductor layers.
Although conducting black matrices are also commonly used in LCD structures (see, for example, U.S. Pat. No. 5,666,177 Hsieh et al.), they are normally allowed to float electrically or, on occasion, may be embedded within the common ITO (indium tin oxide) layer (31 in FIG. 2) so as to reduce the latter""s series resistance (see U.S. Pat. No. 5,721,599 Cheng). Beyond this, no additional uses for the black matrix have thus far been reported. The present invention teaches several additional areas in which the black matrix may be applied without in any way detracting from its light blocking and absorbing capabilities.
It has been an object of the present invention to provide a liquid crystal display that includes thin film transistors and a light-blocking black matrix that is included within the TFT structure.
Another object of the invention has been to utilize said black matrix for the performance of other functions in addition to light blocking.
A further object of the invention has been to utilize the black matrix to facilitate connecting each thin film transistor to one of the transparent conducting blocks.
A still further object of the invention has been to reduce the series resistance of said TFTs in the display.
Yet another object has been to provide a process for the manufacture of said liquid crystal display.
These objects have been achieved by locating the TFTs directly below the spaces between pixels. The black matrix comprises an array of opaque conductive elements with one such element being above each TFT. By using highly conductive material for the black matrix elements their thickness is held to a minimum, thereby minimizing their impact on planarity. Optionally, this highly conductive layer may be laminated with layers of a non-reflective conductor that makes good ohmic contact to silicon. In one embodiment, metal filled via holes are added that connect the TFTs to the transparent conductive blocks by way of the black matrix layer. In another embodiment, the black matrix layer is connected to be in parallel with the gate electrode, thereby reducing the series resistance of the latter.